In various types of rotating equipment having a rotating shaft, thrust bearings are conventionally provided in cooperative engagement with the shaft to accommodate thrust loads to which the shaft may be subjected. Thrust bearings may use liquid lubrication between a rotor mounted to a shaft and stators which are provided together to axially support the rotor and associated shaft when subjected to thrust loads. Such thrust loads may be generated by various causes, such as operational loads encountered during operation of the rotating equipment.
The invention relates to a dry gas thrust bearing for use in various types of rotating equipment. The thrust bearing will preferably be used in equipment such as turbines, expanders, and compressors, although it will be understood that the inventive thrust bearing could be applied to any type of turbomachinery equipment requiring shaft thrust retention. The device could also be installed in equipment used in other applications in which conventional liquid lubricated thrust bearings could not be applied.
The invention comprises a double symmetrical rotating arrangement that provides a shaft mounted rotor which is formed as an annular radial flange projecting radially, preferably from the shaft. The rotor has opposed bearing faces which face in opposite axial directions, and the thrust bearing further includes a pair of thrust ring stators which include respective bearing faces which face axially towards the rotor bearing faces. During shaft rotation, the rotor bearing faces rotate relative to the stator bearing faces. The stators preferably are axially movable relative to the rotor and include biasing means preferably formed as a spring package which biases the stators towards the rotor bearing faces. While the stators are axially movable and biased, the rotor could also use axially movable rings to define the rotor bearing faces and in turn, the stators in such an instance could be axially fixed.
Some or possibly all of these bearing faces include hydrodymanic lift features which hydrodynamically generate a dry gas fluid film between the rotor and stator bearing faces during relative rotation thereof. In other words, the fluid film is generated during rotation of the rotor relative to the stationary stator bearing faces which occurs during shaft rotation. The hydrodynamic lift features rely on a dry gas film being generated between the opposing bearing faces to axially separate each stator from the opposing rotor. Sufficient film stiffness is generated between the bearing faces to counterbalance the thrust load produced from a rotating shaft. One inventive feature of the present invention includes the retention method for the biasing means which provides a flexible mounting, preferably for each bearing stator.
The biasing means preferably is formed as an inventive double spring arrangement which acts on the respective back face of each stator to normally bias each stator toward the rotor while still permitting each stator to move axially away from the rotor as a result of hydrodynamic lift generated by the fluid film and as a result of thrust loads imparted by the shaft which shaft may displace axially under such thrust loads.
Prior to and during shaft axial movement, the double springs are arranged in series to provide two functions. First, a lightly loaded spring engages the stator thrust ring assembly during zero thrust applications. Second, as the shaft begins to rotate and encounter thrust, a heavier stiffer spring, greater than the thrust load, quickly engages. The stiffer spring allows the stator to remain flexibly mounted during dynamic operation. The dry gas bearing was designed to support various upset thrust excursions during operation including shaft thrusting in the opposite direction.
The inventive thrust bearing provides the double springs stacked in series, wherein these two separate springs provide separate spring rates. One spring delivers a low spring force while the second spring delivers a high spring force.
The smaller spring rate allows the dry gas thrust bearing to maintain a light pre-load during initial start-up. This light spring load provides the ability to generate the necessary film stiffness for supporting the thrust load developed during dynamic operation. The thrust load tends to increase as a function of increasing speed.
When the thrust load exceeds the loading of the smaller spring, this smaller spring becomes non-functional and the heavy (increased stiffness) spring engages thereby supporting the stationary thrust bearing face. This transition occurs within very limited axial shaft travel. Preferably, the approximate shaft travel is less than 0.254 mm (0.010″) for shaft size diameters less than 2.54 cm (1 inch).
The double spring feature is a unique concept that provides a buffer period during static to dynamic operation. During this transition, the bearing faces generate the necessary film stiffness for adequate thrust load support.
Other objects and purposes of the invention, and variations thereof, will be apparent upon reading the following specification and inspecting the accompanying drawings.
Certain terminology will be used in the following description for convenience in reference only and will not be limiting. The words “up”, “down”, “right” and left” will designate directions in the drawings to which reference is made. The words “in” and “out” will refer to directions toward and away from, respectively, the geometric center of the device and designated parts thereof. The words “proximal” and “distal” will refer to the orientation of an element with respect to the device. Such terminology will include derivatives and words of similar import.